Skew measurement apparatus, skew measurement method, recording media and test apparatus

ABSTRACT

Provided is a skew measurement apparatus, including sampling sections that each sample one of a plurality of signals under measurement having a cycle T, a waveform reconfiguring section that shapes a reconfigured waveform having the cycle T by rearranging ordinal ranks of sample values of the signal under measurement sampled by each sampling section, a distribution generating section that generates a timing distribution of edges in the reconfigured waveform of the corresponding signal under measurement, and a skew calculating section that calculates skew between the signals under measurement being compared based on the timing distribution of each signal under measurement.

BACKGROUND

1. Technical Field

The present invention relates to a skew measurement apparatus, a skew measurement method, a recording medium, and a test apparatus. In particular, the present invention relates to a skew measurement apparatus, a skew measurement method, a recording medium, and a test apparatus for measuring skew of a plurality of signal under measurement using a digital comparator such as a voltage comparator.

2. Related Art

Skew refers to the timing difference between a plurality of signals output from a plurality of signal sources. Conventional skew measurement methods use a time interval analyzer or a frequency counter to statistically estimate the skew. For example, Wavecrest Corp., Jitter Analysis Clock Solutions, 1998 discloses an exemplary skew measurement using a time interval analyzer. As another skew measurement example, U.S. Pat. No. 7,127,018 discloses a technique for obtaining instantaneous phases of clock signals under measurement and obtaining the clock skew from the initial phase difference between these instantaneous phases.

The clock skew measurement method using the time interval analyzer, however, has dead time after each single measurement during which another measurement cannot be performed, and therefore a long sampling time is required to achieve the desired measurement accuracy. Since waveform measurement accuracy of approximately 8 bits is required to accurately calculate the initial phase of the instantaneous phase of the clock signal under measurement using the analysis technique disclosed in U.S. Pat. No. 7,127,018, this technique cannot be applied to a semiconductor test apparatus that samples a waveform with a 1-bit comparator.

SUMMARY

Therefore, it is an object of an aspect of the innovations herein to provide a skew measurement apparatus, a skew measurement method, a recording medium, and a test apparatus, which are capable of overcoming the above drawbacks accompanying the related art. The above and other objects can be achieved by combinations described in the independent claims. The dependent claims define further advantageous and exemplary combinations of the innovations herein.

According to a first aspect related to the innovations herein, one exemplary skew measurement apparatus may include a skew measurement apparatus, including sampling sections that each sample one of a plurality of signals under measurement having a cycle T, a waveform reconfiguring section that shapes a reconfigured waveform having the cycle T by rearranging ordinal ranks of sample values of the signal under measurement sampled by each sampling section, a distribution generating section that generates a timing distribution of edges in the reconfigured waveform of each signal under measurement, and a skew calculating section that calculates skew between the signals under measurement being compared based on the timing distribution of each signal under measurement. The sampling sections may each sample the corresponding signal under measurement having the cycle T a certain number of times N while the signal under measurement repeats for M cycles, where M and N are coprime. The waveform reconfiguring section may rearrange initial ordinal ranks k of the sample values sampled by the sampling sections to have reconfigured ordinal ranks i by inputting the initial ordinal ranks k into an expression i=k·M mod N, where k is expressed by integers from 0 to N−1, and may use these rearranged sample values to shape the reconfigured waveform having the cycle T.

The skew calculating section may calculate the skew as a difference between expected values of the timing distributions of the signals under measurement being compared or to be a minimum difference or a maximum difference between timing distribution ends of the signals under measurement being compared. The distribution generating section may include a unit-interval waveform generating section that generates a plurality of unit-interval waveforms by dividing the reconfigured waveform into a plurality of unit intervals, an aggregate value calculating section that calculates an aggregate value at each phase in each of the plurality of unit-interval waveforms by adding together the sample values at the same phase of each unit-interval waveform, and a difference calculating section that calculates a difference between each pair of aggregate values at adjacent phases. The distribution generating section may generate the timing distribution based on the differences calculated by the difference calculating sections.

The distribution generating section may further include a waveform inverting section that inverts the unit-interval waveforms generated by the unit-interval waveform generating section, and the aggregate value calculating section may add the sample values of the unit-interval waveforms having rising edges to the sample values of the unit-interval waveforms having falling edges that are inverted by the waveform inverting section, or may add the sample values of the unit-interval waveforms having falling edges to the sample values of the unit-interval waveforms having rising edges that are inverted by the waveform inverting section. The skew measurement apparatus may further include a prescribed bit unit-interval waveform group selecting section that selects the unit-interval waveforms generated by the unit-interval waveform generating section at prescribed bit intervals in the signal under measurement, and the aggregate value calculating section may add together the sample values of each unit-interval waveform in the group selected by the prescribed bit unit-interval waveform group selecting section. The aggregate value calculating section may add together the sample values of the unit-interval waveforms having rising edges or the sample values of the unit-interval waveforms having falling edges.

The skew measurement apparatus may further include an identical data sequence unit-interval waveform group selecting section that selects, from among the unit-interval waveforms generated by the unit-interval waveform generating section, a group of unit-interval waveforms having identical data sequences in the signal under measurement immediately prior thereto, and the aggregate value calculating section may add together the sample values of each unit-interval waveform in the group selected by the identical data sequence unit-interval waveform group selecting section. The skew measurement apparatus may further include a prescribed bit unit-interval waveform group selecting section that selects the unit-interval waveforms generated by the unit-interval waveform generating section at prescribed bit intervals in each signal under measurement, and the aggregate value calculating section may add the sample values of the unit-interval waveforms having rising edges to the sample values of the unit-interval waveforms having falling edges that are inverted by the waveform inverting section in the group of unit-interval waveforms selected by the prescribed bit unit-interval waveform group selecting section.

The configurations described above can be adopted in a similar manner to provide a skew measurement method, a recording medium storing thereon a program used by the skew measurement apparatus, and a test apparatus that supplies a test signal to a device under test to test the device under test. The test apparatus may further include a skew notification section that that provides notification concerning the skew calculated by the skew calculating section, which is the skew between the signals under measurement, and a frequency characteristic adjusting section that adjusts a frequency characteristic of the test signal to decrease the skew according to the skew as notified by the skew notification section.

The summary clause does not necessarily describe all necessary features of the embodiments of the present invention. The present invention may also be a sub-combination of the features described above. The above and other features and advantages of the present invention will become more apparent from the following description of the embodiments taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary configuration of a skew measurement apparatus 100.

FIG. 2 shows an exemplary configuration of a waveform processing section 140.

FIG. 3 shows an exemplary configuration of a statistical value calculating section 146.

FIG. 4 shows a method for shaping the reconfigured waveform.

FIG. 5 shows exemplary sample values of an input waveform Xs [k].

FIG. 6 shows an example of a reconfigured waveform of the signal under measurement 10.

FIG. 7 shows an example of the unit-interval waveforms in the signal under measurement 10.

FIG. 8 shows an example in which a portion of the unit-interval waveforms in the signal under measurement 10 are inverted.

FIG. 9 shows an example of an aggregate waveform obtained by adding together the unit-interval waveforms in the signal under measurement 10.

FIG. 10 shows an example of a differential waveform obtained by differentiating the aggregate waveform.

FIG. 11 shows an example of an edge timing distribution.

FIG. 12 shows the order of the process performed by the skew measurement apparatus 100.

FIG. 13 shows another exemplary configuration of the distribution generating section 144.

FIG. 14 shows the order of the process performed by another embodiment.

FIG. 15 shows an exemplary configuration of a semiconductor test apparatus 1600.

FIG. 16 shows an exemplary hardware configuration of the skew measurement apparatus 100.

FIG. 17 shows an exemplary configuration of a jitter measurement apparatus 2100.

FIG. 18 shows an exemplary configuration of a rise/fall time measurement apparatus 2000.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, some embodiments of the present invention will be described. The embodiments do not limit the invention according to the claims, and all the combinations of the features described in the embodiments are not necessarily essential to means provided by aspects of the invention.

FIG. 1 is a block diagram showing an exemplary configuration of a skew measurement apparatus 100. The skew measurement apparatus 100 receives a plurality of signals under measurement, such as a signal under measurement 10 and a signal under measurement 20, as input and outputs a value of the skew and a value of the deterministic skew between the signal under measurement 10 and the signal under measurement 20. The skew represents the timing difference between a plurality of signals output from a plurality of signal sources. The skew may be obtained from the rising edge timing between the signals. The deterministic skew represents the deterministic component of the skew. If it is assumed that the signals contain no jitter, the deterministic skew can be obtained by comparing the rising edge timings of the signals.

As shown in FIG. 1, the skew measurement apparatus 100 is provided with a plurality of sampling sections 110 and a plurality of skew evaluating section 130 s, and may also be provided with a memory 120. The skew measurement apparatus 100 may be provided with a number of sampling sections equal to the number of signal under measurement. The skew evaluating section 130 includes a waveform processing section 140 and a skew calculating section 150. The skew evaluating section 130 may include a plurality of waveform processing sections 140. The skew evaluating section 130 may include a number of waveform processing sections 140 equal to the number of sampling sections 110.

The following describes the skew measurement apparatus 100 as having two sampling sections 110 and two waveform processing sections 140, but the skew measurement apparatus 100 is not limited to this configuration. In the present embodiment, one of the sampling sections 110 receives the signal under measurement 10 and the other sampling section 110 receives the signal under measurement 20. One of the waveform processing sections 140 may correspond to one of the sampling sections 110, and the other waveform processing section 140 may correspond to the other sampling section 110.

The sampling sections 110 sample the signal under measurement 10 and the signal under measurement 20 having a cycle T. The sampling sections 110 may sample the signal under measurement 10 and the signal under measurement 20 having the cycle T a certain number of times N while the signal under measurement 10 and the signal under measurement 20 repeat for M cycles, where M and N are coprime. The following describes one of the sampling sections 110 receiving the signal under measurement 10, but the sampling section 110 that receives the signal under measurement 20 can sample the signal under measurement 20 in the same manner. The sampling section 110 samples the input signal under measurement 10 with a sampling cycle Ts, based on a strobe timing 118, for example, to shape an input waveform X_(S)[k] of the input signal under measurement 10. Here, k represents the ordinal rank of the sample values sampled by the sampling section 110, and may be an integer from 0 to N−1. The integer k represents the initial sampling ordinal rank and the value of k in the input waveform X_(S)[k] increases in the sampled order.

The sampling section 110 is an AD converter or a voltage comparator (sometimes referred to hereinafter as an “ADC”), for example. The sampling section 110 may be a digital comparator or a waveform digitizer. The resolution of the ADC is 1 bit or 1.6 bits, for example. When the ADC with a 1-bit resolution is used, the sample values of the input waveform X_(S)[k] may be expressed by binary logic values, such as 0 and 1. When the ADC with a 1.6-bit resolution is used, the sample values of the input waveform X_(S)[k] may be expressed by three values. The sampling section 110 is not limited to sampling the signal under measurement 10 only while the signal under measurement 10 having the cycle T is repeating for M cycles. For example, the sampling section 110 may sample the signal under measurement 10 while the signal under measurement 10 is repeating for 5M cycles.

The memory 120 may store sampling results and the like of the sampling section 110. For example, the memory 120 stores sample values of the input waveforms X_(S)[k] corresponding to the signal under measurement 10 and the signal under measurement 20 respectively, obtained by the sampling section 110 sampling the signal under measurement 10 and the signal under measurement 20, in association with k indicating the initial ordinal rank with an integer from 0 to N−1.

The skew evaluating section 130 calculates an edge timing distribution for each of the signal under measurement 10 and the signal under measurement 20, and outputs the skew between the signal under measurement 10 and the signal under measurement 20. The skew evaluating section 130 may output the deterministic skew. The skew evaluating section 130 may obtain the skew between the signal under measurement 10 and the signal under measurement 20 by reading the input waveforms X_(S)[k] corresponding to the signal under measurement 10 and the signal under measurement 20 that are stored in the memory 120.

Upon receiving the input waveform X_(S)[k] acquired from the memory 120, the waveform processing section 140 outputs the statistical value of the edge timing distribution of the signal under measurement 10, for example. If the input waveform X_(S)[k] of the signal under measurement 20 is input, the waveform processing section 140 outputs the statistical value of the edge timing distribution of the signal under measurement 20 in the same manner as signal under measurement 10. The waveform processing section 140 has a waveform reconfiguring section 142, a distribution generating section 144, and a statistical value calculating section 146.

The waveform reconfiguring section 142 rearranges the order of the sample values of the input waveform X_(S)[k] sampled by the sampling section 110 to shape a reconfigured waveform X_(R)[i] having a cycle T. Here, i represents the rearranged ordinal rank. The relationship between the rearranged ordinal rank i and the initial ordinal rank k is shown by Expression 1, and i may be an integer from 0 to N−1. When M is expressed by M=nN+1, where n is any natural number, the rearranging process described above can be omitted since i=k.

i=(k·M)mod N  Expression 1

When the signal under measurement 10 or the signal under measurement 20 has the cycle T and M and N are coprime, the sampling section 110 can sample the signal under measurement 10 or the signal under measurement 20 with a cycle Te that is shorter than a sampling cycle Ts. This effective sampling cycle Te is shown by Expression 2. Hereinafter, the effective sampling cycle Te is sometimes referred to as an “equivalent sampling time interval”.

Te=T/N=Ts/M  Expression 2

The distribution generating section 144 generates a timing distribution of the edges in the reconfigured waveform X_(R)[i]. This distribution is sometimes referred to hereinafter as an “edge timing distribution,” and the edge timing distribution is an example of the timing distribution. The edge timing distribution may be a PDF of the timing of each edge in the signal under measurement 10.

The statistical value calculating section 146 calculates the statistical value of the edge timing distribution. The statistical value calculating section 146 may calculate an expected value of the edge timing distribution and a distribution width of the edge timing distribution as the statistical value. Here, the expected value may be an average value. The distribution width may be an indicator of the range of the data variation, such as a variance, a standard deviation, a quartile deviation, an RMS value, a peak-to-peak value, or a half width. In the present embodiment, the phrase “distribution ends” refers to both ends of the distribution width, which are the minimum value and the maximum value of the edge timing distribution, for example. The distribution ends may be obtained using the average value of the edge timing distribution and the distribution width of the standard deviation or the like.

The skew calculating section 150 may calculate the skew between the signal under measurement 10 and the signal under measurement 20 being compared based on the edge timing distributions of each of these signals. The skew calculating section 150 may output the skew as the difference between the expected value of the edge timing distribution of the signal under measurement 10 and the expected value of the edge timing distribution of the signal under measurement 20. The skew calculating section 150 may calculate the difference between the distribution ends of the edge timing distribution of the signal under measurement 10 and the distribution ends of the edge timing distribution of the signal under measurement 20 and output the difference between the minimum values or the difference between the maximum values of the distribution ends as the skew.

The skew measurement apparatus 100 may be hardware or may be software that causes a processor, not shown, to execute a process. The processor may be a CPU or a computer controlling the process. The functions of the elements included in the skew evaluating section 130 are not strictly separated. For example, the statistical value calculating section 146 may have the functions of the distribution generating section 144 and the skew calculating section 150 may have the functions of the waveform processing section 140.

FIG. 2 is a block diagram showing an exemplary configuration of the waveform processing section 140. The waveform processing section 140 includes the waveform reconfiguring section 142, the distribution generating section 144, and the statistical value calculating section 146, as described above. The distribution generating section 144 includes a unit-interval waveform generating section 202, an aggregate value calculating section 208, and a difference calculating section 210, and may also include a unit-interval waveform group selecting section 204 and a waveform inverting section 206. The following describes an example in which the waveform processing section 140 receives the input waveform X_(S)[k] of the signal under measurement 10, but the waveform processing section 140 performs an identical process if the input waveform X_(S)[k] of the signal under measurement 20 is received.

Upon receiving the reconfigured waveform X_(R)[i] from the waveform reconfiguring section 142, the unit-interval waveform generating section 202 divides X_(R)[i] into a prescribed number of unit intervals Tu to generate a plurality of unit-interval waveforms X_(p)[m]. The unit-interval waveform generating section 202 desirably divides the reconfigured waveform X_(R)[i] such that each unit-interval waveform X_(p)[m] has an edge roughly in the center.

Here, the coefficient Lu and the bit period Tb, which represents the bit time interval of the signal under measurement 10, are used to express Tu as Tu=Lu·Tb. If Lu=1, for example, Tu may be equal to the bit period Tb representing the bit time interval of the signal under measurement 10, and Tu is desirably chosen to be a value such that Tu/Te is equal to a positive integer. It is desirable that Lu be a positive integer. The ordinal rank of a certain unit-interval waveform X_(p)[m] in the plurality of unit-interval waveforms X_(p)[m] is represented by p. For example, the number Nu of unit-interval waveforms X_(p)[m] included in the sampled signal under measurement 10 is used to express p as an integer from 1 to Nu. The number Nu may be greater than N, depending on the sampling period of the signal under measurement 10.

The ordinal rank of the sample values of the unit-interval waveforms X_(p)[m] is represented by m. Hereinafter, m is sometimes referred to as the “ordinal rank after division”. The relation between the ordinal rank after division m and the reconfigured ordinal rank i is shown by Expression 3. In Expression 3, Tu/Te represents the number of sample values included in each unit-interval waveform X_(p)[m]. The ordinal rank after division m is represented by an integer from 0 to (Tu/Te)−1.

m=i mod(Tu/Te)=i mod(N·Tu/T)  Expression 3

The unit-interval waveform group selecting section 204 may generate a unit-interval waveform group by selecting unit-interval waveforms X_(p)[m] having a prescribed feature from among the plurality of unit-interval waveforms X_(p)[m] generated by the unit-interval waveform generating section 202 dividing the reconfigured waveform X_(R)[i]. For example, the unit-interval waveform group selecting section 204 selects only the unit-interval waveforms X_(p)[m] having a rising edge or only the unit-interval waveforms X_(p)[m] having a falling edge. If the ADC with a 1-bit resolution is used, the unit-interval waveform group selecting section 204 may select the unit-interval waveforms X_(p)[m] having a rising edge by selecting only the unit-interval waveforms X_(p)[m] in which the first sample value is logic L. In the same manner, the unit-interval waveform group selecting section 204 may select the unit-interval waveforms X_(p)[m] having a falling edge by selecting only the unit-interval waveforms X_(p)[m] in which the first sample value is logic H.

The waveform inverting section 206 may invert the unit-interval waveforms X_(p)[m] generated by the unit-interval waveform generating section 202. By doing this, the waveform inverting section 206 can invert the unit-interval waveforms X_(p)[m] having falling edges to be unit-interval waveforms X_(p)[m] having rising edges. It should be noted that the edge timing of the edges in the unit-interval waveforms X_(p)[m] does not change even when the unit-interval waveform group selecting section 204 inverts the unit-interval waveforms X_(p)[m].

The aggregate value calculating section 208 adds the sample values of identical phases in the plurality of unit-interval waveforms X_(p)[m] to obtain an aggregate value for each phase. For example, the aggregate value calculating section 208 may calculate the aggregate value at each ordinal rank after division m for each unit-interval waveform X_(p)[m] by adding together the sample values at each of the ordinal ranks after division m, which has a relation to the reconfigured ordinal ranks i as expressed by m=i mod (N·Tu/T), included in the unit-interval waveform X_(p)[m]. The aggregate value calculating section 208 may add the sample values of the unit-interval waveforms X_(p)[m] having rising edges and the inverted sample values of the unit-interval waveforms X_(p)[m] having falling edges that are inverted by the waveform inverting section 206. The aggregate value calculating section 208 may instead add the sample values of the unit-interval waveforms X_(p)[m] having falling edges and the inverted sample values of the unit-interval waveforms X_(p)[m] having rising edges that are inverted by the waveform inverting section 206.

The aggregate value calculating section 208 may instead add only the sample values of the unit-interval waveforms X_(p)[m] having rising edges, or may add only the sample values of the unit-interval waveforms X_(p)[m] having falling edges. For example, the skew of the rising edges and of the falling edges is different if the signal under measurement 10 is a clock signal with a duty ratio other than 50%. Even if the signal under measurement 10 is such a signal, the skew measurement apparatus 100 described above can accurately measure both the skew of the falling edges and the skew of the rising edges respectively.

The difference calculating section 210 calculates the difference between two aggregate values in adjacent phases. For example, the difference calculating section 210 calculates the difference between aggregate values having adjacent ordinal ranks after division m for the aggregate value at each ordinal rank after division m in the plurality of unit-interval waveforms X_(p)[m]. The distribution generating section 144 generates the timing distribution based on the differences calculated by the difference calculating section 210. For example, the distribution generating section 144 may generate the edge timing distribution in which the differences calculated by the difference calculating section 210 are associated with the ordinal rank after division m.

FIG. 3 is a block diagram showing an exemplary configuration of the statistical value calculating section 146. The statistical value calculating section 146 calculates the statistical value of the edge timing distribution generated by the distribution generating section 144. The statistical value calculating section 146 may have an expected value calculating section 302 and a distribution width calculating section 304. The expected value calculating section 302 may calculate the expected value of the edge timing distribution. The expected value of the edge timing distribution is an average value, for example.

The distribution width calculating section 304 may calculate the distribution width of the edge timing distribution. By doing this, the distribution width calculating section 304 can calculate the width of the fluctuation of the edge timing distribution on the time axis. The distribution width calculating section 304 may have a standard deviation calculating section 312, a maximum value calculating section 314, a minimum value calculating section 316, and a peak-to-peak calculating section 318. The standard deviation calculating section 312 calculates the standard deviation of the edge timing distribution, as an example of the distribution width. The maximum value calculating section 314 calculates the maximum value of the edge timing distribution, as an example of the distribution width. The minimum value calculating section 316 calculates the minimum value of the edge timing distribution, as an example of the distribution width. The peak-to-peak calculating section 318 may calculate the peak-to-peak value, which is an example of the distribution width. The peak-to-peak calculating section 318 may calculate the peak-to-peak value by subtracting the minimum value calculated by the minimum value calculating section 316 from the maximum value calculated by the maximum value calculating section 314.

By adopting the configuration described above, the waveform processing section 140 can generate the edge timing of the signal under measurement 10 based on the sampled input waveform X_(S)[k]. The waveform processing section 140 can also calculate the statistical value of the edge timing distribution by calculating the statistical value of the generated edge timing distribution. The following is a detailed description of the performances of the waveform reconfiguring section 142, the distribution generating section 144, and the statistical value calculating section 146 in the waveform processing section 140.

FIG. 4 shows a method by which the waveform reconfiguring section 142 shapes the reconfigured waveform. FIG. 4 describes the basics of the process performed by the waveform reconfiguring section 142 when the signal under measurement 10 is a 3-bit data signal having a data sequence “101”. In FIG. 4, the thick line near the top represents the signal under measurement 10, the thick line at the bottom represents the reconfigured waveform X_(R)[i], the white circles represent logic L, and the black circles represent logic H. The signal under measurement 10 that is used is a data signal in which the bit period is Tb and the repeating cycle L of the data pattern is 3 bits. The sampling section 110 samples the signal under measurement 10 in synchronization with the repeating period of the signal under measurement 10 with a sampling frequency Ts that is less than the Nyquist frequency. The equivalent sampling time intervals Te are set equal to Tb/3, and the sampling cycle Ts is set equal to 4Te. Here, N=9, M=4, and M and N are coprime.

In FIG. 4, sample 0 represents the point at which the sampling begins, that is, the sampling occurring at the timing k=0. As shown in FIG. 4, the sample value of the input waveform X_(S)[0] at sample 0 was logic H. Sample 1 represents the point reached after the time Ts has passed since sample 0, that is, the sampling occurring at the timing k=1. The sample value of the input waveform X_(S)[1] at sample 1 was logic L. The input waveform X_(S)[k] was obtained by sampling the signal under measurement 10 at the timings k=0 to k=8 in the same manner described above.

The sample values of the input waveform X_(S)[k] obtained by the sampling section 110 through the sampling described above are arranged according to the initial ordinal rank k as sampled by the sampling section 110. The waveform reconfiguring section 142 obtains the reconfigured waveform X_(R)[i] having the cycle T by rearranging the order of the sample values to have the rearranged ordinal rank i. The reconfigured ordinal rank i is obtained from Expression 1. For example, the i corresponding to k=3 is i=(3·4) mod 9=3. Specifically, the sample value sampled at a time when 3Ts has passed since initiation of the sampling is treated in the reconfigured waveform X_(R)[i] as a sample value sampled at a time when 3Te has passed since initiation of the sampling. The waveform reconfiguring section 142 obtains the reconfigured waveform X_(R)[i] by rearranging the sample values from k=0 to k=8 in the same manner.

FIG. 5 shows exemplary sample values of the input waveform X_(S)[k]. FIG. 5 shows an example of the results of a different sampling in which an ADC with a 1-bit resolution samples the signal under measurement 10. In FIG. 5, the signal under measurement 10 is a clock pattern. The sampling frequency Ts is set to be less than the Nyquist frequency and M and N are set to be coprime. As shown in FIG. 5, 80 samples are performed from k=0 to k=79. The following describes the process performed by the distribution generating section 144 and the statistical value calculating section 146 using the input waveform X_(S)[k] of FIG. 5 as an example.

FIG. 6 shows an example of the reconfigured waveform X_(R)[i] obtained by reconfiguring the input waveform X_(S)[k] of FIG. 5. The reconfigured waveform X_(R)[i] has 80 sample values from i=0 to i=79, and the interval between sample values is expressed by the equivalent sampling time interval Te. In the reconfigured waveform X_(R)[i], the bit period Tb is set equal to the unit interval Tu, so that the unit-interval waveform X_(p)[m] repeats eight times.

FIG. 7 shows an example of the unit-interval waveform X_(p)[m] included in the reconfigured waveform X_(R)[i] of FIG. 6. FIG. 7 is used to describe the general process performed by the unit-interval waveform generating section 202. When the bit period Th is set equal to the unit interval Tu and the reconfigured waveform X_(R)[i] is divided into a plurality of unit intervals Tu, eight unit-interval waveforms X_(p)[m] are obtained from p=1 to p=8. As shown in FIG. 7, each unit-interval waveform X_(p)[m] includes ten sample values from m=0 to m=9. The unit-interval waveforms X_(p)[m] at p=1, 3, 5, 7 have rising edges, and the unit-interval waveforms X_(p)[m] at p=2, 4, 6, 8 have falling edges.

FIG. 8 shows an example in which a portion of the unit-interval waveforms shown in FIG. 7 are inverted. FIG. 8 is used to describe the general processes performed by the unit-interval waveform group selecting section 204 and the waveform inverting section 206. The unit-interval waveform group selecting section 204 selects only the unit-interval waveforms X_(p)[m] having falling edges in FIG. 7. The waveform inverting section 206 inverts the logic values of the unit-interval waveforms X_(p)[m] selected by the unit-interval waveform group selecting section 204 to obtain inverted waveforms X_(p, rising)[m]. In this way, the inverted waveforms X_(p, rising)[m] of FIG. 8 all have rising edges. The unit-interval waveform group selecting section 204 may instead select only the unit-interval waveforms X_(p)[m] having rising edges, and the waveform inverting section 206 may invert the logic values of these unit-interval waveforms X_(p)[m] so that all of the inverted waveforms have falling edges.

FIG. 9 shows an example of an aggregate waveform X_(SUM)[m] obtained by adding the unit-interval waveforms of FIG. 8 together. FIG. 9 is used to describe the general process performed by the aggregate value calculating section 208. The aggregate waveform X_(SUM)[m] can be obtained by adding together the logic values of the inverted waveforms X_(p, rising)[m] from p=1 to p=8 at each ordinal rank after division m and calculating the aggregate value at each ordinal rank after division m, for example. In FIG. 9, the aggregate values from X_(SUM)[0] to X_(SUM)[3] are 0, the aggregate value of X_(SUM)[4] is 2, the aggregate value of X_(SUM)[5] is 6, the aggregate values of X_(SUM)[6] to X_(SUM)[9] are 8. From this, it is understood that the edge timing is between m=3 and m=6. It should be noted that since the sample values are expressed as binary logic values when the ADC with a 1-bit resolution is used, the aggregate value is equivalent to a count value.

FIG. 10 shows an example of a differential waveform X_(DIFF)[m] obtained by differentiating the aggregate waveform X_(SUM)[m]. FIG. 10 is used to describe the general process of the difference calculating section 210. The differential waveform X_(DIFF)[M] is obtained by calculating the differences between the aggregate values of the aggregate waveform X_(SUM)[m] at each ordinal rank m and the aggregate values of the aggregate waveform X_(SUM)[m−1] at each ordinal rank m−1 adjacent to a certain ordinal rank m, for example. The differential waveform X_(DIFF)[m] is an example of the edge timing distribution. As shown in FIG. 10, the waveform X_(DIFF)[4] at m=4 is 2, the waveform X_(DIFF)[5] at m=5 is 4, waveform X_(DIFF)[6] at m=6 is 2, and the values of X_(DIFF)[m] at the other ordinal ranks after division m are 0. From this, it is understood that the edge timing is between m=3 and m=6.

FIG. 11 is a block diagram showing an example of the edge timing distribution. FIG. 11 is used to describe the general process performed by the statistical value calculating section 146. The statistical value calculating section 146 may use Expression 4 to calculate an average value E of the edge timing distribution of the signal under measurement 10 based on the differential waveform X_(DIFF)[m]. By doing this, the statistical value calculating section 146 can obtain an average value E₁₀ of the edge timing of the signal under measurement 10. In FIG. 11, E₂₀ represents the average value of the edge timing of the signal under measurement 20. In Expression 4, t_(m) represents a sampling time of the signal under measurement 10 or the signal under measurement 20, and is expressed by t_(m)=m·Te. Furthermore, max(X_(SUM)[m]) represents the maximum value of X_(SUM)[m].

$\begin{matrix} {E = {\sum\limits_{m = 0}^{\frac{Tu}{Te} - 1}{t_{m} \cdot \frac{X_{DIFF}\lbrack m\rbrack}{\max \left( {X_{sum}\lbrack m\rbrack} \right)}}}} & {{Expression}\mspace{14mu} 4} \end{matrix}$

The statistical value calculating section 146 may use Expression 5 to calculate a standard deviation TJ_(RMS). The statistical value calculating section 146 may calculate the maximum value T_(edge, max) and the minimum value T_(edge, min) of the edge timing distribution. The maximum value T_(edge, max) may be calculated as the maximum time t_(m) at which X_(DIFF)[m] is not zero. The minimum value T_(edge, min) may be calculated as the minimum time t_(m) at which X_(DIFF)[m] is not zero. In FIG. 11, T¹⁰ _(edge, max) represents the maximum value of the edge timing distribution of the signal under measurement 10, T²⁰ _(edge, max) represents the maximum value of the edge timing distribution of the signal under measurement 20, T¹⁰ _(edge, min) represents the minimum value of the edge timing distribution of the signal under measurement 10, and T²⁰ _(edge, min) represents the minimum value of the edge timing distribution of the signal under measurement 20. The statistical value calculating section 146 may use Expression 6 to calculate a peak-to-peak value TJ_(PP). In Expression 6, max(t_(m)|x_(DIFF)≠0) represents the maximum time t_(m) at which X_(DIFF)[m] is not zero, and min(t_(m)|x_(DIFF)≠0) represents the minimum time t_(m) at which X_(DIFF)[m] is not zero.

$\begin{matrix} {{TJ}_{RMS} = \sqrt{\sum\limits_{m = 0}^{\frac{Tu}{Te} - 1}\left\{ {\left( {t_{m} - E} \right)^{2} \cdot \frac{X_{DIFF}\lbrack m\rbrack}{\max \left( {X_{sum}\lbrack m\rbrack} \right)}} \right\}}} & {{Expression}\mspace{14mu} 5} \end{matrix}$ TJ _(PP) =T _(edge,max) −T _(edge,min)

TJ _(PP)=max(t _(m) |x _(DIFF)≠0)−min(t _(m) |x _(DIFF)≠0)  Expression 6

The following uses FIG. 11 to describe the general process performed by the skew calculating section 150. The skew calculating section 150 may obtain the skew τ_(skew) between the signal under measurement 10 and the signal under measurement 20 by calculating the difference between the average value E₁₀ of the edge timing of the signal under measurement 10 and the average value E₂₀ of the edge timing of the signal under measurement 20, or the absolute value of this difference. The skew calculating section 150 may calculate the maximum values or the minimum values of the timing distribution ends of the signal under measurement 10 and the signal under measurement 20. For example, the skew calculating section 150 uses Expression 7 to obtain the maximum skew value τ_(skew, max). In Expression 7, max(T¹⁰ _(edge, max), T²⁰ _(edge, max)) represents the greater of the two values T¹⁰ _(edge, max) and T²⁰ _(edge, max), and min(T¹⁰ _(edge, min) T²⁰ _(edge, min)) represents the lesser of the two values T¹⁰ _(edge, min) and T²⁰ _(edge, min).

τ_(skew,max=max() T ¹⁰ _(edge,max) ,T ²⁰ _(edge,max)−min() T ¹⁰ _(edge,min) ,T ²⁰ _(edge,min))  Expression 7

The skew calculating section 150 may calculate the skew τ_(skew) as the difference between any one of the average value, the minimum value, or the maximum value of the edge timing distribution of the signal under measurement 10 and any one of the average value, the minimum value, or the maximum value of the edge timing distribution of the signal under measurement 20. For example, the skew calculating section 150 may calculate the skew τ_(skew) to be the difference between E₁₀ and T²⁰ _(edge, max) FIGS. 5 to 11 described the operation of the skew evaluating section 130 with regards to a clock signal, but the operation of the skew evaluating section 130 is not limited to a clock signal. The skew evaluating section 130 can calculate the skew τ_(skew) in the same manner for other types of periodic signals.

FIG. 12 is a flow chart showing the order of the process performed by the skew measurement apparatus 100. FIG. 12 is used to describe the performance of the skew measurement apparatus 100. At S1202, the sampling sections 110 sample the signal under measurement 10 and the signal under measurement 20 having a cycle T to obtain the sample values of the respective input waveforms X_(S)[k]. One of the sampling sections 110 samples the signal under measurement 10 with a condition that the sampling is performed N times while the signal under measurement 10 repeats for M cycles, where M and N are coprime. The other sampling section 110 samples the signal under measurement 20 under the same condition. The signal under measurement 10 and the signal under measurement 20 are desirably sampled at substantially the same time and with substantially the same sampling frequency Ts. The thus obtained sample values may be stored in the memory 120. The process described hereinafter from S1204 to S1220 is applied to the respective input waveforms X_(S)[k] of the signal under measurement 10 and the signal under measurement 20.

At S1204, the waveform reconfiguring section 142 reconfigures the input waveform X_(S)[k] to generate the reconfigured waveform X_(R)[i]. The waveform reconfiguring section 142 may reconfigure the input waveform X_(S)[k] by acquiring the sample values stored in the memory 120 and rearranging the initial ordinal ranks k to be the reconfigured ordinal ranks i using the calculation i=(k·M) mod N. At S1206, the unit-interval waveform generating section 202 divides the reconfigured waveform X_(R)[i] into unit intervals Tu to obtain a plurality of unit-interval waveforms X_(p)[m].

At S1208, a target is selected for calculating the edge timing distribution from among the plurality of unit-interval waveforms X_(p)[m] obtained at S1206. The selection mode may be a mode for selecting only the unit-interval waveforms X_(p)[m] having rising edges, a mode for selecting only the unit-interval waveforms X_(p)[m] having falling edges, or a mode for selecting all of the unit-interval waveforms X_(p)[m].

If the mode used at S1208 is a mode selecting only the unit-interval waveforms X_(p)[m] having rising edges ((a) at S1208), the unit-interval waveform group selecting section 204 selects only the unit-interval waveforms X_(p)[m] having rising edges, from among the plurality of unit-interval waveforms X_(p)[m] obtained at S1206, to generate the unit-interval waveform group at S1210.

On the other hand, if the mode used at S1208 is a mode selecting only the unit-interval waveforms X_(p)[m] having falling edges or for selecting all of the unit-interval waveforms X_(p)[m] ((b) at S1208), the unit-interval waveform group selecting section 204 selects only the unit-interval waveforms X_(p)[m] having falling edges, from among the plurality of unit-interval waveforms X_(p)[m] obtained at S1206, to generate the unit-interval waveform group at S1212. At S1214, the waveform inverting section 206 inverts the waveform of the group of unit-interval waveforms having falling edges.

At step S1216, the aggregate value calculating section 208 adds the logic values of the unit-interval waveforms X_(p)[m], which are selected by the unit-interval waveform group selecting section 204 at S1208 according to the mode, at each ordinal rank after division m to obtain the aggregate waveform X_(SUM)[m]. At S1218, the difference calculating section 210 differentiates the aggregate waveform X_(SUM)[m] to obtain the differential waveform X_(DIFF)[m]. The differential waveform X_(DIFF)[m] can be obtained by calculating the amount of change in the aggregate waveform X_(SUM)[m] for each equivalent sampling time interval Te, for example. The amount of change in the aggregate waveform X_(SUM)[m] can be obtained from the differences between the aggregate values of the aggregate waveform X_(SUM)[m] at each ordinal rank m and the aggregate values of the aggregate waveform X_(SUM)[m−1] at each ordinal rank m−1 adjacent to a certain ordinal rank m, for example.

At S1220, the statistical value calculating section 146 calculates the statistical value of the edge timing distribution based on the differential waveform X_(DIFF)[m] obtained at S1218. The statistical value of the edge timing distribution may be the average value E, the standard deviation TJ_(RMS), the maximum value T_(edge, max), the minimum value T_(edge, min), or the peak-to-peak value TJ_(PP) of the edge timing distribution. Lastly, at S1222, the skew evaluating section 130 calculates the skew as the difference between the average value E₁₀ of the edge timing distribution of the signal under measurement 10 and the average value E₂₀ of the edge timing distribution of the signal under measurement 20, and ends the process.

FIG. 13 is a block diagram showing another exemplary configuration of the distribution generating section 144. A distribution generating section 1344 has an identical data sequence unit-interval waveform group selecting section 1304 instead of a unit-interval waveform group selecting section 204, but is otherwise identical to the distribution generating section 144. The identical data sequence unit-interval waveform group selecting section 1304 selects a group of unit-interval waveforms having identical data sequences of the sample values in the signal under measurement 10 or the signal under measurement 20 immediately prior thereto, from among the unit-interval waveforms X_(p)[m] generated by the unit-interval waveform generating section 202. The aggregate value calculating section 208 adds together the logic values of the sample values for each unit-interval waveform in the group selected by the identical data sequence unit-interval waveform group selecting section 1304, instead of for each unit-interval waveform in the group selected by the unit-interval waveform group selecting section 204.

FIG. 14 is a flow chart showing the order of the process performed by the distribution generating section 1344 of FIG. 13. FIG. 14 is used to describe a process of using the identical data sequence unit-interval waveform group selecting section 1304 to calculate the skew when the signal under measurement 10 repeats an L-bit data pattern. Here, L represents a positive integer. The process from S1402 to S1406 is identical to the process from S1202 to S1206, and therefore a description is omitted. The process from S1410 to S1414 is identical to the process from S1216 to S1220, except for the processing of the identical data sequence unit-interval waveform group selected at S1408, and therefore a description of the identical portions is omitted. The following describes the process performed for the input waveform X_(S)[k] corresponding to the signal under measurement 10, but the same process from S1402 to S1414 is performed for the input waveform X_(S)[k] corresponding to the signal under measurement 20.

At S1408 in the present embodiment, the identical data sequence unit-interval waveform group selecting section 1304 generates the group of identical data sequence unit-interval waveforms, each having immediately prior data sequences that are identical to the data sequence immediately prior to the edge being measured. The identical data sequence unit-interval waveform group can be generated as described hereinafter, for example. First, the reconfigured waveform X_(R)[i] is divided into bit periods. The identical data sequence unit-interval waveform group selecting section 1304 then selects a specific edge to be measured from among the edges in the signal under measurement 10. The identical data sequence unit-interval waveform group selecting section 1304 selects only the unit-interval waveforms having the same data sequence for L-bits immediately prior thereto as that of the specific edge, from among all of the unit-interval waveforms, and extracts the selected unit waveforms. In this way, the data sequence unit-interval waveform group selecting section 1304 can generate the group of identical data sequence unit-interval waveforms, each having immediately prior data sequences that are identical to the data sequence immediately prior to the edge being measured.

From S1410 to S1414, the edge timing distribution and the average value of the edge timing distribution are calculated for each unit-interval waveform included in the identical data sequence unit-interval waveform group obtained at S1304, in the same manner as the process from S1216 to S1220. In this way, the average value of the edge timing of the specific edge can be calculated. The average value of the edge timing for each of the plurality of edges in the signal under measurement 10 can be calculated by the process from S1408 to S1414 as shown above. The process from S1408 to S1414 may be performed for all of the edges in the signal under measurement 10.

At S1416, the skew calculating section 150 may obtain the skew by calculating the difference between the average value of the edge timing of the specific edge of the signal under measurement 10 and the average value of the edge timing of the corresponding specific edge of the signal under measurement 20 obtained at S1414. In this way, the error between the timing of the specific edge and the ideal edge timing can be calculated. The skew calculating section 150 may obtain the skew through a variety of comparison combinations between any one of the statistical values of the edge timing distribution of the signal under measurement 10 and any one of the statistical values of the edge timing distribution of the signal under measurement 20.

The skew calculating section 150 may calculate the skew between a plurality of edges in the signal under measurement 10 and a plurality of edges in the signal under measurement 20 if an edge timing distribution is desired for a plurality of edges. In this way, the skew calculating section 150 can generate a skew sequence by arranging the skew values in a time series. The skew calculating section 150 may obtain the skew by calculating the statistical value of this skew sequence. The skew calculating section 150 may calculate an RMS value as the statistical value. The skew evaluating section 130 outputs the skew obtained by the skew calculating section 150 and ends the process.

The present embodiment describes an example using the identical data sequence unit-interval waveform group selecting section 1304 instead of the unit-interval waveform group selecting section 204, but both the identical data sequence unit-interval waveform group selecting section 1304 and the unit-interval waveform group selecting section 204 may also be used. Instead of the identical data sequence unit-interval waveform group selecting section 1304, a prescribed bit unit-interval waveform group selecting section may be used that selects unit-interval waveforms generated by the unit-interval waveform generating section 202 at prescribed bit intervals in the signal under measurement 10 or the signal under measurement 20. The aggregate value calculating section 208 may add the sample values of the unit-interval waveforms having falling edges that are inverted by the waveform inverting section 206 to the sample values of the unit-interval waveforms having rising edges for each prescribed bit unit-interval waveform in the group selected by the prescribed bit unit-interval waveform group selecting section. Furthermore, the skew measurement apparatus 100 may include both the prescribed bit unit-interval waveform group selecting section and the unit-interval waveform group selecting section 204.

The above description discloses a skew measurement method including the steps of sampling each of a plurality of signal under measurement having a cycle T, shaping reconfigured waveforms having the cycle T by rearranging the order of the sample values of each signal under measurement obtained at the sampling step, generating timing distributions of a portion of the edges in the reconfigured waveform of each signal under measurement, and calculating the skew between the signals under measurement being compared based on the timing distribution of each signal under measurement. Furthermore, a skew measurement method is disclosed for, at the sampling step described above, sampling each signal under measurement a certain number of times N while the signal under measurement having the cycle T repeats for M cycles, where M and N are coprime. Yet further, a skew measurement method is disclosed for, at the reconfiguring step described above, rearranging the order of the sample values obtained at the sampling step to have reconfigured ordinal ranks i by inputting the initial ordinal ranks k into the expression i=k·M mod N, where k is expressed by integers from 0 to N−1, and using these rearranged sample values to shape reconfigured waveforms having the cycle T.

FIG. 15 is a block diagram showing an exemplary configuration of a semiconductor test apparatus 1600. The semiconductor test apparatus 1600 is an example of a test apparatus. The semiconductor test apparatus 1600 provides a test signal 1602 to a device under test 60 to test the device under test 60. The semiconductor test apparatus 1600 is provided with a signal generating section 1610 and a signal measuring section 1620. The signal generating section 1610 generates the test signal 1602. The signal measuring section 1620 measures an output signal 1604 and an output signal 1605 of the device under test 60. The output signal 1604 and the output signal 1605 have a cycle T. The signal generating section 1610 includes a signal generator 1612 and a frequency characteristic adjusting section 1614. The signal measuring section 1620 includes the skew measurement apparatus 100 and a skew notification section 1622.

The signal generator 1612 generates the test signal 1602. The frequency characteristic adjusting section 1614 may adjust the frequency characteristic of the test signal 1602 to decrease the skew of the output signal 1604 in accordance with the skew of the output signal 1604 measured by the signal measuring section 1620. The frequency characteristic adjusting section 1614 may adjust the frequency characteristic of the test signal 1602 according to the skew indicated by notification from the skew notification section 1622. For example, if the skew indicated by the notification from the skew notification section 1622 is greater than a prescribed value, the frequency characteristic adjusting section 1614 adjusts an equalizer, not shown, to enhance the high frequency component of the test signal 1602. Each sampling section 110 of the skew measurement apparatus 100 samples the output signal 1604 or the output signal 1605 a certain number of times N while the output signal being sampled repeats for M cycles, where M and N are coprime. The skew notification section 1622 notifies the signal generating section 1610 concerning the skew of the output signal 1604 or the output signal 1605, which is the statistical value calculated by the statistical value calculating sections 146 of the skew measurement apparatus 100.

FIG. 16 shows an exemplary hardware configuration of the skew measurement apparatus 100. More specifically, FIG. 16 shows an exemplary hardware configuration in which the skew measurement apparatus 100 is an electronic information processing apparatus such as a computer. The skew measurement apparatus 100 is provided with a CPU peripheral section, an input/output section, and a legacy input/output section. The CPU peripheral section includes a CPU 1805, a RAM 1820, a graphic controller 1875, and a display apparatus 1880 connected to each other by a host controller 1882. The input/output section includes a communication interface 1830, a hard disk drive 1840, and a CD-ROM drive 1860, all of which are connected to the host controller 1882 by an input/output controller 1884. The legacy input/output section includes a ROM 1810, a flexible disk drive 1850, and an input/output chip 1870, all of which are connected to the input/output controller 1884.

The host controller 1882 is connected to the RAM 1820 and is also connected to the CPU 1805 and graphic controller 1875 accessing the RAM 1820 at a high transfer rate. The CPU 1805 operates to control each section based on programs stored in the ROM 1810 and the RAM 1820. The graphic controller 1875 acquires image data generated by the CPU 1805 or the like on a frame buffer disposed inside the RAM 1820 and displays the image data in the display apparatus 1880. Alternatively, the graphic controller 1875 may internally include the frame buffer storing the image data generated by the CPU 1805 or the like.

The input/output controller 1884 connects the hard disk drive 1840 serving as a relatively high speed input/output apparatus, the communication interface 1830, and the CD-ROM drive 1860 to the host controller 1882. The hard disk drive 1840 stores the programs and data used by the CPU 1805. The communication interface 1830 is connected to a network communication apparatus 1898 to receive the programs or the data. The CD-ROM drive 1860 reads the programs and data from a CD-ROM 1895 and provides the read information to the hard disk drive 1840 and the communication interface 1830 via the RAM 1820.

The input/output controller 1884 is connected to the ROM 1810, and is also connected to the flexible disk drive 1850 and the input/output chip 1870 serving as a relatively high speed input/output apparatus. The ROM 1810 stores a boot program performed when the skew measurement apparatus 100 starts up, a program relying on the hardware of the skew measurement apparatus 100, and the like. The flexible disk drive 1850 reads the programs or data from a flexible disk 1890 and supplies the read information to the hard disk drive 1840 and the communication interface 1830 via the RAM 1820. The input/output chip 1870 connects the flexible disk drive 1850 to each of the input/output apparatuses via, a parallel port, a serial port, a keyboard port, a mouse port, or the like.

The programs performed by the CPU 1805 are stored on a recording medium such as the flexible disk 1890, the CD-ROM 1895, or an IC card and are provided by the user. The programs stored on the recording medium may be compressed or uncompressed. The programs are installed on the hard disk drive 1840 from the recording medium, are read by the RAM 1820, and are performed by the CPU 1805. The programs performed by the CPU 1805 cause the skew measurement apparatus 100 to function as the sampling section 110, the waveform reconfiguring section 142, the distribution generating section 144, and the statistical value calculating section 146 described in relation to FIGS. 1 to 15.

The programs shown above may be stored in an external storage medium. In addition to the flexible disk 1890 and the CD-ROM 1895, an optical recording medium such as a DVD or PD, a magneto-optical medium such as an MD, a tape medium, a semiconductor memory such as an IC card, or the like can be used as the recording medium. Furthermore, a storage apparatus such as a hard disk or a RAM disposed in a server system connected to the Internet or a specialized communication network may be used as the storage medium and the programs may be provided to the skew measurement apparatus 100 via the network.

FIG. 17 is a block diagram showing an exemplary configuration of a jitter measurement apparatus 2100. The jitter measurement apparatus 2100 receives the signal under measurement 10 as input and outputs a jitter value of the signal under measurement 10. The jitter measurement apparatus 2100 may output a deterministic jitter value, such as a data-dependent jitter value, as well as a frequency component of the deterministic jitter. The jitter measurement apparatus 2100 is provided with the sampling section 110 and a jitter calculating section 2130, and may also be provided with the memory 120. The jitter calculating section 2130 calculates an edge timing distribution of the signal under measurement 10 and outputs the jitter value of the signal under measurement 10. The jitter calculating section 2130 may obtain the jitter value of the signal under measurement 10 by reading the input waveform X_(S)[k] stored in the memory 120.

The jitter calculating section 2130 includes the waveform processing section 140, and may also include a ideal edge timing calculating section 2150, an error sequence generating section 2160, an error sequence statistical value calculating section 2170, and a Fourier transform section 2180. The error sequence generating section 2160 may generate an error sequence obtained by arranging, in a time-series, the errors between the expected values of the edge timing distribution calculated by the statistical value calculating section 146 and the ideal edge timing calculated by the ideal edge timing calculating section 2150. The error sequence statistical value calculating section 2170 may calculate the statistical value of the error sequence. The Fourier transform section 2180 may calculate the frequency component of the error sequence by performing a Fourier transform of the error sequence. If the errors included in the error sequence are not spaced uniformly on the time axis, the Fourier transform section 2180 may interpolate the jitter value at a bit boundary having no jitter value based on the previous and subsequent jitter values to generate the error sequence in which the errors are spaced uniformly on the time axis, and then perform the Fourier transform of the thus generated error sequence.

As a specific example, the jitter measurement apparatus 2100 is provided with a sampling section that samples a signal under measurement having a cycle T for a certain number of cycles N while the signal under measurement repeats for M cycles, where M and N are coprime; a waveform reconfiguring section that rearranges the order of the sample values of the signal under measurement sampled by the sampling section to have reconfigured ordinal ranks i by inputting the initial ordinal ranks k into the expression i=k·M mod N, where k is expressed by integers from 0 to N−1, and that uses these rearranged sample values to shape a reconfigured waveform having the cycle T; a distribution generating section that generates timing distributions of the edges in the reconfigured waveform; and a statistical value calculating section that calculates the statistical value of the timing distribution of the signal under measurement.

FIG. 18 is a block diagram showing an exemplary configuration of a rise/fall time measurement apparatus 2000. The rise/fall time measurement apparatus 2000 is an example of the signal measurement apparatus. The rise/fall time can express the edge transition time of the signal under measurement 10, and can be obtained by calculating the timing difference of the 20% level cross-point and the 80% level cross-point of the signal amplitude, for example. The rise/fall time measurement apparatus 2000 can measure the rise/fall time of the signal under measurement 10 by sampling the signal under measurement 10 substantially simultaneously with two comparators having different threshold values to calculate the difference between the average value of the edge timings at the 80% level cross-point and the average value of the edge timings at the 20% level cross-point.

As shown in FIG. 18, the rise/fall time measurement apparatus 2000 is provided with the sampling section 110, the sampling section 112, the memory 120, and a rise/fall time evaluating section 2030. The sampling section 112 has a configuration identical to that of the sampling section 110, except for having a different threshold value. The rise/fall time measurement apparatus 2000 has a configuration identical to that of the skew measurement apparatus 100, except that the rise/fall time measurement apparatus 2000 measures a single signal under measurement 10 with both the sampling section 110 and the sampling section 112. The rise/fall time evaluating section 2030 includes the waveform processing section 140 and a rise/fall time calculating section 2050. The rise/fall time evaluating section 2030 may include a plurality of waveform processing sections 140.

The waveform processing section 140 may calculate the average value of the edge timings at the 20% level cross-points of the signal under measurement 10 and the average value of the edge timings at the 80% level cross-points of the signal under measurement 10. The average value of the edge timings is an example of the statistical value of the edge timing distribution. The rise/fall time calculating section 2050 can measure the rise time or the fall time of the signal under measurement 10 by calculating the difference between the average value of the edge timings at the 20% level cross-points of the signal under measurement 10 and the average value of the edge timings at the 80% level cross-points of the signal under measurement 10.

As a specific example, the rise/fall time measurement apparatus 2000 is provided with sampling sections that respectively sample the signal under measurement with two different threshold values, each sampling section sampling the signal under measurement for a certain number of cycles N while the signal under measurement having a cycle T that repeats for M cycles, where M and N are coprime; waveform reconfiguring sections that rearrange the order of the sample values of the signal under measurement sampled by the sampling sections to have reconfigured ordinal ranks i by inputting the initial ordinal ranks k into the expression i=k·M mod N, where k is expressed by integers from 0 to N−1, and that use these rearranged sample values to shape a reconfigured waveform having the cycle T; distribution generating sections that generate timing distributions of the edges in the reconfigured waveform for each threshold value; and a calculating section that calculates the rise time or fall time of the signal under measurement based on the timing distributions for the threshold values.

While the embodiments of the present invention have been described, the technical scope of the invention is not limited to the above described embodiments. It is apparent to persons skilled in the art that various alterations and improvements can be added to the above-described embodiments. It is also apparent from the scope of the claims that the embodiments added with such alterations or improvements can be included in the technical scope of the invention.

As made clear from the above, the embodiments of the present invention can be adopted to realize a skew measurement apparatus, a skew measurement method, a recording medium, and a test apparatus for measuring the skew of a plurality of signals under measurement using a digital comparator such as a voltage comparator. 

1. A skew measurement apparatus, comprising: sampling sections that each sample one of a plurality of signals under measurement having a cycle T; a waveform reconfiguring section that shapes a reconfigured waveform having the cycle T by rearranging ordinal ranks of sample values of the signal under measurement sampled by each sampling section; a distribution generating section that generates a timing distribution of edges in the reconfigured waveform of each signal under measurement; and a skew calculating section that calculates skew between the signals under measurement being compared based on the timing distribution of each signal under measurement.
 2. The skew measurement apparatus according to claim 1, wherein the sampling sections each sample the corresponding signal under measurement having the cycle T a certain number of times N while the signal under measurement repeats for M cycles, where M and N are coprime.
 3. The skew measurement apparatus according to claim 2, wherein the waveform reconfiguring sections each rearrange initial ordinal ranks k of the sample values sampled by the sampling sections to have reconfigured ordinal ranks i by inputting the initial ordinal ranks k into an expression i=k·M mod N, where k is expressed by integers from 0 to N−1, and use these rearranged sample values to shape the reconfigured waveform having the cycle T.
 4. The skew measurement apparatus according to claim 1, wherein the skew calculating section calculates the skew based on a difference between expected values of the timing distributions of the signals under measurement being compared or based on a minimum difference or a maximum difference between timing distribution ends of the signals under measurement being compared.
 5. The skew measurement apparatus according to claim 1, wherein the distribution generating section includes: a unit-interval waveform generating section that generates a plurality of unit-interval waveforms by dividing the reconfigured waveform into a plurality of unit intervals; an aggregate value calculating section that calculates an aggregate value at each phase in each of the plurality of unit-interval waveforms by adding together the sample values at the same phase of each unit-interval waveform; and a difference calculating section that calculates a difference between each pair of aggregate values at adjacent phases, and the distribution generating section generates the timing distribution based on the difference calculated by the difference calculating section.
 6. The skew measurement apparatus according to claim 5, wherein the distribution generating section further includes a waveform inverting section that inverts the unit-interval waveforms generated by the unit-interval waveform generating section, and the aggregate value calculating section adds the sample values of the unit-interval waveforms having rising edges to the inverted sample values of the unit-interval waveforms having falling edges that are inverted by the waveform inverting section, or adds the sample values of the unit-interval waveforms having falling edges to the inverted sample values of the unit-interval waveforms having rising edges that are inverted by the waveform inverting section.
 7. The skew measurement apparatus according to claim 5, wherein the aggregate value calculating section adds together the sample values of the unit-interval waveforms having rising edges or the sample values of the unit-interval waveforms having falling edges.
 8. The skew measurement apparatus according to claim 5, further comprising an identical data sequence unit-interval waveform group selecting section that selects, from among the unit-interval waveforms generated by the unit-interval waveform generating section, a group of unit-interval waveforms having identical data sequences in the signal under measurement immediately prior thereto, wherein the aggregate value calculating section adds together the sample values of each unit-interval waveform in the group selected by the identical data sequence unit-interval waveform group selecting section.
 9. The skew measurement apparatus according to claim 5, further comprising a prescribed bit unit-interval waveform group selecting section that selects the unit-interval waveforms generated by the unit-interval waveform generating section at prescribed bit intervals in each signal under measurement, wherein the aggregate value calculating section adds together the sample values of each unit-interval waveform in the group selected by the prescribed bit unit-interval waveform group selecting section.
 10. The skew measurement apparatus according to claim 6, further comprising a prescribed bit unit-interval waveform group selecting section that selects the unit-interval waveforms generated by the unit-interval waveform generating section at prescribed bit intervals in each signal under measurement, wherein the aggregate value calculating section adds the sample values of the unit-interval waveforms having rising edges to the inverted sample values of the unit-interval waveforms having falling edges that are inverted by the waveform inverting section in the group of unit-interval waveforms selected by the prescribed bit unit-interval waveform group selecting section.
 11. A skew measurement method, comprising the steps of: sampling a plurality of signals under measurement having a cycle T; shaping reconfigured waveforms having the cycle T by rearranging ordinal ranks of sample values of each signal under measurement sampled at the sampling step; generating timing distributions of edges in the reconfigured waveform of each signal under measurement; and calculating skew between the signals under measurement being compared based on the timing distribution of each signal under measurement.
 12. The skew measurement method according to claim 11, wherein the sampling step includes sampling each signal under measurement having the cycle T a certain number of times N while the signals under measurement repeats for M cycles, where M and N are coprime.
 13. The skew measurement method according to claim 12, wherein the reconfiguring step includes rearranging initial ordinal ranks k of the sample values obtained at the sampling step to have reconfigured ordinal ranks i by inputting the initial ordinal ranks k into an expression i=k·M mod N, where k is expressed by integers from 0 to N−1, and using these rearranged sample values to shape the reconfigured waveforms having the cycle T.
 14. A recording medium storing thereon a program that, when used by a skew measurement apparatus, causes the skew measurement apparatus to function as: sampling sections that each sample one of a plurality of signals under measurement having a cycle T a certain number of times N while the signal under measurement repeats for M cycles, where M and N are coprime; a waveform reconfiguring section that rearranges initial ordinal ranks k of the sample values sampled by each sampling section to have reconfigured ordinal ranks i by inputting the initial ordinal ranks k into an expression i=k·M mod N, where k is expressed by integers from 0 to N−1, and uses these rearranged sample values to shape reconfigured waveforms having the cycle T; a distribution generating section that generates timing distributions of edges in the reconfigured waveform of each signal under measurement; and a skew calculating section that calculates skew between the signals under measurement being compared based on the timing distribution of each signal under measurement.
 15. A test apparatus that provides a test signal to a device under test to test the device under test, comprising: a signal generating section that generates the test signal; and a signal measuring section that measures a plurality of output signals of the device under test, wherein the signal measuring section includes, sampling sections that each sample one of a plurality of output signals having a cycle T a certain number of times N while the output signal repeats for M cycles, where M and N are coprime; a waveform reconfiguring section that rearranges initial ordinal ranks k of the sample values sampled by the sampling sections to have reconfigured ordinal ranks i by inputting the initial ordinal ranks k into an expression i=k·M mod N, where k is expressed by integers from 0 to N−1, and uses these rearranged sample values to shape reconfigured waveforms having the cycle T; a distribution generating section that generates a timing distribution of edges in the reconfigured waveform of each output signal; and a skew calculating section that calculates skew between the output signals being compared based on the timing distribution of each output signal.
 16. The test apparatus according to claim 15, wherein the signal measuring section further includes a skew notification section that notifies the signal generating section concerning the skew calculated by the skew calculating section, as a skew between the output signals, and the signal generating section includes a frequency characteristic adjusting section that adjusts a frequency characteristic of the test signal to decrease skew between the output signals according to the skew as notified by the skew value notification section. 